SALEKHARD, Russia — Russia announced on Tuesday that it had opened a piracy investigation against the crew of a Greenpeace ship after its activists scaled an offshore oil platform in the Arctic last week. The step signaled that the authorities intended to act decisively to thwart more protests against Russia’s ambitious plans to expand energy exploration in the region.

The Greenpeace ship, the Arctic Sunrise, was seized by Russian border guards in international waters in the Pechora Sea last Thursday, a day after several members of the crew tried to board the Prirazlomnaya platform, which is operated by the Russian state energy giant Gazprom.